This invention relates to the control of insects such as common houseflies (Musca domestica), fruit flies (Drosophila melanogaster), mosquitoes (Culex pipiens) and other similar insects in the vicinity of an insecticide-containing device.
Heretofore, insect-combatting devices, such as pest strips and the like, comprised of a PVC resin having a dispersion of the insecticide dimethyl 2,2-di-chlorovinyl phosphate, commonly known as DDVP or by its trademark Vapona, have been widely used for the purpose of controlling flying insects such as houseflies, mosquitoes and the like in the vicinity of the device. However, DDVP has been reported to have an objectionable depressing effect on the plasma and red cell cholinesterase at least in animals which effect is particularly acute at high concentrations which are produced during the first few days after a pest strip has first been exposed to the atmosphere. This is believed due to the fact that the liberation rate of DDVP from presently available DDVP-containing pest strips is not uniform but rather is higher during the first few days after activation, i.e., removal of the pest strip from the packing and exposing it to the atmosphere. There are also indications that DDVP may be harmful to humans. Pest strips containing DDVP have been banned in Holland. Moreover, the aforementioned initial high liberation rate represents an unduly rapid loss of insecticide and creates an upper limit on the period that DDVP is liberated at a rate sufficient to effectively control pests. DDVP also has been found to possess a high degree of residual toxicity in the area of the device, apparently from adsorption of the DDVP vapors in walls, floors, ceilings, curtains, rugs, and the like. Even after a DDVP-containing pest strip is removed from a room environment, residual DDVP vapors can often be detected for several days thereafter.
It has also been suggested to utilize other insecticides such as naled (1,2-dibromo-2,2-dichloroethyl dimethyl phosphate) in an insect-combatting device such as a pest strip. The preparation of naled is described in U.S. Pat. No. 2,971,882 to Osmonson et al. PVC resin-naled combinations have been proposed for use as an insecticide of a general nature in French Pat. No. 1,568,198, issued Apr. 14, 1969, and in U.S. patent application Ser. No. 85,445, filed Jan. 30, 1961 (abandoned, but accessible to the public), and corresponding British Pat. No. 955,350. Netherlands published application No. 6,610,279 discloses fly strips composed of PVC-naled as well as PVC-DDVP combinations which are stated to have such high insecticide release rates as to require an outer laminate layer to retard the insecticide release. U.S. Pat. No. 3,344,021 discloses PVC-naled combinations for use as an anthelmintic composition.
A number of problems have been encountered in providing a commercially satisfactory PVC-naled combination for use in an insect-combatting device. First, there must be a sufficient amount of naled released to provide effective control of the insects in the vicinity of the device. Contrary to statements in the prior art disclosures, it has been found that release rates for naled are very much less than the release rates for DDVP. Naled has a low vapor pressure of about 2 .times. 10.sup..sup.-4 mm. Hg. at 20.degree. C. as compares to that for DDVP of 1.2 .times. 10.sup..sup.-2 to be thus only about 1.7% of the vapor pressure of DDVP.
It has further been found that the inclusion of an insecticide such as naled in a synthetic resin matrix in amounts sufficient to control insects for a commercially acceptable time leads to exudation of liquid insecticide (or "spew") on the surface of the device. These liquid droplets pose serious environmental and aesthetic problems as well as significantly decreasing the effective life of the device.
A further unexpected problem found with a PVC-naled composition was the tendency of the resin to decompose during the shaping process. For example, unsatisfactory results were obtained in early tests where naled was substituted for DDVP in PVC combinations employed in extrusion apparatus used for making PVC-DDVP pet collars known in the art. Burning and charring of the extrudate were found to occur during curing of the collars, and the finished collar underwent an unexplainable reduction in the naled concentration as compared with the naled concentration in the original mixture.